The formulation of a coating particularly suitable for exterior metal surfaces such as for automobiles is complex. The coating must remain essentially unchanged in appearance over a period of several years while being exposed to a variety of weather conditions.
The two major components of the coating are the vehicle and the pigment. The vehicle can vary widely in stability properties. The pigment can also vary considerably in stability properties, and in some cases, there is some interaction between the pigment and the vehicle when subjected to actual weather conditions.
Titanium dioxide is a most important pigment in such coatings, and there is a large quantity of information available on methods and techniques to increase the stability properties of titanium dioxide. A good review of the literature on this subject is available in "Titanium, Its Occurrence, Chemistry, and Technology", Jelks Barksdale, Ronald Press, New York, Second Edition, 1966, pages 533-567.
However, titanium dioxide coated mica nacreous pigment is a more complicated entity than pigmentary titanium dioxide with respect to stability properties in a coating exposed to weather conditions. Thus, methods and techniques used to stabilize pigmentary titanium dioxide are either ineffective or insufficient to provide stability for titanium dioxide coated mica platelets because there are reactions that involve the mica-titanium dioxide interface as well as the titanium dioxide alone.
The major effort in the past for imparting stability to pigmentary titanium dioxide has been for coatings on exterior wood surfaces, known as trade sales paints. It has been well recognized that solar radiation in the near ultraviolet region was the major cause of film degradation. It is only in recent years that it has been recognized that the outdoor weathering of a film which leads to its deterioration is much more complicated than just its exposure to near ultraviolet light from the sun. Deterioration of a film coating is due to the combination of the near ultraviolet light from the sun, moisture, and heat. Each of these agents may have a different effect on the vehicle, on the pigment, or on both, and at the present time, it is not possible to clearly delineate the various mechanisms of the chemical and physical reactions which take place. Unquestionably, there are also many interactions.
The use of titanium dioxide coated mica platey pigments in coatings on metal for exterior purposes has been faced with two major obstacles. One was an incomplete recognition of the weather conditions that lead to film breakdown, and the other was inadequate treatment of the pigment to provide it with improved stability.
Of utmost importance is the recognition today that not only is the film affected by the several agents outlined above, but also the changes in these conditions can lead to the degradation of the coating film. This recognition is suitably described in the paper, "Correlation of Laboratory to Natural Weathering", by G. W. Grossman, Journal of Coatings Technology, Vol. 49, No. 633, pp. 45-54, Oct. 1977. Hereinafter, the effects of cyclic variations in near ultraviolet radiation, moisture, and temperature will be referred to as "weatherability stress".
The industry standard weatherability testing is to subject the coated metal panels to outdoor Florida weather for up to two years. The conditions there are most severe, since the daily cycle includes the night with lower temperature and high humidity, possibly some water condensation on the panels, change in intense sunlight in the morning along with substantial temperature increases, the possibility of liquid water on the panel from rain in the afternoon followed by sunlight again and decreasing humidity, and finally the night again with falling temperatures and increasing humidity.
Obviously, it would greatly facilitate the testing of pigments for weatherability stress by using simulated weather conditions in the laboratory. It has been found that this can be accomplished by using a laboratory instrument, the Q-U-V Cyclic Ultraviolet Weathering Tester, which provides cyclical weather conditions for coated metal panels so that in a 24 hour cycle, variations in near ultraviolet light, water spray, and temperature are presented to the panels. It has been found that after about one month exposure in the Q-U-V, various coated metal panel samples can be ranked in the same order as those exposed in outdoors southern Florida for two years.
It should be recognized that this type of testing is much more relevant and complete than much of the laboratory testing that has been carried out in the past. The Fade-Ometer testing is inadequate because only radiation exposure is involved. The Cleveland Humidity Cabinet is insufficient for this type of testing since the panels are subjected only to heat and moisture. Other instruments which present either moisture or ultraviolet radiation alone are inadequate for the same reasons. Further, the near ultraviolet radiation should present a substantial intensity in the wavelength region from 310-390 nm, and some ultraviolet light instruments have relatively poor intensities in this region. Finally, the near ultraviolet radiation, moisture, and temperature must be presented to the test panels on a cyclical basis, since the most severe aspect of this testing is that of the weatherability stress.
Although a great deal of work has been done in the past on modifying pigmentary titanium dioxide so as to improve its various stability properties, the testing has been done under single exposure conditions such as near ultraviolet radiation, heat, or moisture. Testing for each one of these conditions alone is inadequate in predicting the weatherability behavior of the coated panel. When titanium dioxide coated mica pigments are incorporated in the film, correlation between actual weatherability and any individual stability test just cited is poor.
Using the Q-U-V Cyclic Ultraviolet Weathering Tester for testing panels having coatings containing titanium dioxide coated mica, the ranking of resistance to change is excellent when compared to southern Florida outdoor exposure.
Well known or conventional titanium dioxide coated mica nacreous pigments, for example, those prepared according to Linton, U.S. Pat. No. 3,087,828, show very poor weatherability stress behavior when the pigment is incorporated in a suitable vehicle, coated on a metal panel, and subjected to testing in the Q-U-V. Conventional titanium dioxide coated mica nacreous pigments have received no special treatment to render them resistant to change under weatherability stress, and in addition, the titanium dioxide is in the anatase crystal form.
Treating pigmentary titanium dioxide with chromium compounds in order to impart to them improved stability properties is known. For example New, Journal Oil and Colour Chemists' Association, 20, 352, 1937, showed the use of chromium oleate in paints pigmented with titanium dioxide to increase the stability of the film. Treating the pigment particles directly with the chromium compound before incorporation into the vehicle was also found to be effective. New, U.S. Pat. No. 2,242,320, used chromium naphthenate as a surface coating on TiO.sub.2. Nelson, U.S. Pat. No. 2,346,322, used deposition on calcined titanium dioxide to improve resistance to chalking and discoloration, in a combination of 0.5% chromium as the oxide, 2.0% zirconium silicate, and 1.0% alumina were used. Schaumann, U.S. Pat. Nos. 2,226,142, and 2,062,137, added a chromium compound to titanium dioxide pigment before calcination, and Coffelt, 2,045,836, shows the precipitation of titanium dioxide in the presence of chromic acid to form a titanium dioxide pigment containing chromate ion. Dawson, 2,232,168, provides for the coating of calcined titanium dioxide with small amounts of hydrous oxides of aluminum (0.25-2% as Al.sub.2 O.sub.3) and chromium (0.01-0.2% as Cr.sub.2 O.sub.3) followed by drying.
There is one example in the prior art of titanium dioxide coated mica receiving a chromium III compound treatment for improved stability, that is Jackson, U.S. Pat. No. 3,832,208. In this patent, the chromium compound treatment is limited to only one specific compound, methacrylatochromic chloride, and the effectiveness of the treatment is attained only if this compound remains essentially unhydrolyzed. Also, the treatment is intended to impart good resistance to change in a film containing the pigment when it is exposed to humidity conditions only.
It is the object of this invention to provide an exterior grade TiO.sub.2 coated mica nacreous pigment which displays high luster and has a high resistance to weatherability stress. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.